Cyclophosphamide is widely used in the treatment of cancer and autoimmune diseases such as lupus nephritis. However, there is considerable variability in the response to cyclophosphamide treatment. Cyclophosphamide is a pro-drug that requires initial activation by CYP liver enzymes. Recent clinical studies have indicated a possible role of one CYP enzyme, CYP2C19 in this activation step. This enzyme has a genetic polymorphism (variants which lack functional activity) and people who have inherited these variants are poor metabolisers of certain drugs.